The ABR service class has been defined by the ATM Forum as one of the five service classes in an ATM network. In an explicit-rate mode of ABR service in an ATM network, a real-time determination may be made as to the amount of bandwidth that each ABR circuit may utilize. The amount of bandwidth allocated to each ABR circuit is known as the Allowed Cell Rate (ACR). The ACR may be transmitted to the source of each circuit via special control cells known as resource management (RM) cells. The format of RM cells and the principles governing their generations and usage are specified by the ATM Forum Traffic Management 4.0 standard. There are two types of RM cells, Forward RM cells (FRM) which travel from the source to the destination, and Backward RM cells (BRM) which travel from the destination back to the source. Specifically, the Allowed Cell Rate is marked into the BRM cells because they are received by the source with a smaller delay than FRM cells. Also, according to the standard, the FRM cell contains a Current Cell Rate (CCR) field which is set by the source to indicate to the ATM network the cell rate at which the source is currently transmitting traffic. The intention of the CCR field is to assist the network in determining the ACR for all ABR circuits.
One key to a successful implementation of ABR service is the ATM network's capability to quickly and fairly adjust the allowed cell rate (ACR) for each ABR circuit when the level of congestion within the network changes. Accordingly, improved algorithms are desirable for varying the allowed cell rate (ACR) in ATM networks for each ABR circuit.